1. Field of the Invention
The invention resides in the field of arc light sources and particularly relates to cathode assemblies for such sources.
2. Description of the Prior Art
Arc lights wherein an electrical discharge takes place in a gap between an anode and cathode are well known in the prior art. In many cases, the stability of the arc path in these lights is of little consequence. However, in some applications, particularly spectrometers, path continuity and uniformity is important for accurate measurements.
In such spectrometers, electrical discharge devices of various types are used to vaporize and excite samples for optical analysis. Among these devices is the direct current plasma arc which is favored for its high degree of sensitivity and simplicity of construction. A major drawback of this discharge technique in spectrometers is a relative lack of stability which is reflected in imprecision of analytical results.
A primary cause of this instability is the tendency of the discharge at the cathode electrode to wander over the surface of the electrode tip. This wander around the tip is seen as a shifting in the position of the arc path or column. The most common attempt to overcome this difficulty has been to use a sharply pointed electrode of reduced cross-section, typically about 0.040 inches. This reduces but does not completely eliminate the problem. Further, the use of a small cross-section requires an electrode material with a very high melting temperature, such as tungsten alloys. These metals result in additional problems. Tungsten, for example, upon excitation emits at approximately 4000 separate wavelengths in the popular and analytically useful region of 2000-6000 A.degree.. This substantially increases the probability of spectral interferences in the analytical result.
Other solutions involve controlling the flow of a current conducting inert gas plasma in the arc path in a manner that mechanically restricts the shape of the gas column such that it will not reflect changes in the position of the cathode discharge. This technique is disclosed in U.S. Pat. No. 4,060,708 by J. P. Walters. In contrast to the prior art, the present invention solves the problem of arc path wander by the use of a unique cathode assembly as disclosed below.